In 2007 we dissected one toxic issue. The horror story continues, but can we still learn from our mistakes?

NEW YORK (Fortune) -- Back two years ago when the mortgage meltdown was heating up, we wrote an article called "Junk Mortgages Under the Microscope" dissecting a particularly wretched
mortgage-backed securities issue peddled by Goldman Sachs.

We wanted to show how these complex securities really worked and how Moody's and S&P, the rating agencies, aided and abetted the process by giving two-thirds of an issue backed by
ultra-risky second mortgages the same safety rating they gave to U.S. Treasury securities.

We thought this was a cautionary tale -- but it's turned into a horror story. All the tranches of this issue, GSAMP-2006 S3, that were originally rated below AAA have defaulted. Two of
the three original AAA -rated tranches (French for "slices") are facing losses of about 90%, and even the "super senior," safer-than-mere-AAA slice is facing losses of 25%. How could this
happen? And what lessons can we take away from it?

Let's revisit the way this security was put together, and how and why it fell apart. And for the first time, we can even estimate the value -- low -- of the mortgages backing it, thanks
to a new service called ABSNet Loan HomeVal.

Our tale begins in April 2006, when Goldman Sachs (GS, Fortune 500) sold $494 million of securities to institutional investors seeking
yields somewhat above those that were available on U.S. Treasuries or high-rated corporate bonds.

It was an especially hinky offering, because it was backed by second mortgages rather than by traditional first mortgages. A first mortgage rarely becomes completely worthless, because a
house is usually worth something.

But often all it takes is a decline of 20% in a home's value to wipe out a second mortgage, which is typically piled on top of an 80% first mortgage. In our case, borrowers' stated equity
in their homes averaged less than 1% -- 0.71%, to be precise. Even that was doubtless overstated because a majority of the mortgages were low-documentation and no-documentation.

Despite these problems, the formulas used by Moody's and S&P allowed Goldman to market the top three slices of the security -- cleverly called A-1, A-2 and A- 3 -- as AAA rated. That
meant they were supposedly as safe as U.S. Treasury securities.

But of course they weren't. More than a third of the loans were on homes in California, then a superhot market, now a frigid one. Defaults and rating downgrades began almost immediately.
In July 2008, the last piece of the issue originally rated below AAA defaulted -- it stopped making interest payments. Now every month's report by the issue's trustee, Deutsche Bank,
shows that the old AAAs -- now rated D by S&P and Ca by Moody's -- continue to rot out.

As of Oct. 26, date of the most recent available trustee's report, only $79.6 million of mortgages were left, supporting $159.9 million of bonds. In other words, each dollar of bonds had
a claim on less than 50¢ of mortgages.

But even worse, those mortgages aren't worth anything like their $79.6 million of face value, according to ABSNet Loan HomeVal. ABSNet, unveiled in October, combines a database from
Lewtan Technologies of Waltham, Mass., that has a list of every mortgage underlying every mortgage-backed issue, with data from Collateral Analytics of Honolulu, which tracks individual
home values. It gives you a snapshot of the value of the collateral backing a mortgage security.

As of Sept. 26 -- a slightly different date from what we're using above -- ABSNet valued the remaining mortgages in our issue at a tad above 20% their face value. Now, watch this math. If
the mortgages are worth 20% of their face value and each dollar of mortgages supports more than $2 of bonds, it means that the remaining bonds are worth maybe 10% of face value.

If all the originally AAA -rated bonds were the same, they'd all be facing losses of 90% or so in value. However, they weren't the same. The A-1 "super senior" tranche was entitled to get
all the principal payments from all the borrowers until it was paid off in full. Then A-2 and A-3 would share the repayments, then repayments would move down to the lower-rated issues.

But under the security's rules, once the M-1 tranche -- the highest-rated piece of the issue other than the A tranches -- defaulted in July 2008, all the A's began sharing in the
repayments. The result is that only about 28% of the original A-1 "super seniors" are outstanding, compared with more than 98% of A-2 and A-3. If you apply a 90% haircut, the losses work
out to about 25% for the "super seniors," and about 90% for A-2 and A-3.

This was an especially bad issue, which we picked (on advice of some bond mavens who aren't competitors of Goldman Sachs) precisely because it was so awful. According to Bloomberg LP,
recent trades in the A's were at less than 7% of face value. So the market is saying the losses are even greater than our estimates.

Goldman and Moody's declined to discuss this security. S&P told us that it had toughened its standards in 2005 and had discontinued rating second-mortgage securities in 2008. "Had we
anticipated fully the severity of the declines in these markets at the time we issued our original ratings, many of those ratings would have been different," a spokesman said.

Now to the investment lessons: The first is, Don't put your faith in rating agencies, even though some branches of the federal government, including the Federal Reserve, use ratings to
determine whether certain securities qualify as collateral under federal loan programs to financial institutions.

Our problem is that if things change for the worse after the original rating comes out, the agencies' response is, "Oops, sorry about that," and they revise the ratings down after you've
already taken a hit. When lawsuits arrive, the agencies say all they did was issue an opinion that's protected under the First Amendment, therefore they're not liable.

The second lesson is, No matter how fancy the name is on the offering statement -- Goldman Sachs, the calumny being heaped on it lately notwithstanding, is still Wall Street's alpha
outfit -- you're on your own if the issue heads south.

The final lesson is, Beware of the dangers of bottom fishing. It would have been tempting to buy this security when the original AAA paper traded down to the low two-digits -- but any
buyer that did that is sitting on big losses.

Just as things often rise further than you think they will and stay there longer, they can also fall further than you think and keep on falling. Remember that when someone tells you that
something is so cheap that it has nowhere to go but up.